Adhesive article having core/sheath structure

ABSTRACT

The invention relates to adhesive articles provided with an elongated core which comprises a fibrous web and with a sheath comprising an adhesive composition, which are particularly suitable for bonding or stiffening a variety of substrates.

The present invention relates to an adhesive article which comprises anelongated core and a sheath which comprises an adhesive composition. Theelongated core comprises a fibrous web, which, in a particularembodiment, may be a non-woven fibrous web. Such adhesive articles canbe designed for distributing and transferring stress within and betweensubstrates to which they are bonded and/or can provide good gap fillingproperties. Thus, the adhesive articles are particularly useful inbonding or stiffening components e.g. in motor vehicles.

Adhesives and sealers used in automotive production are classified asstructural, semi-structural and non-structural, depending on their bondstrength and elasticity. They are usually homogeneous pastes, liquids ortapes of different gap fill capacity, tensile strength and modulus,which build up strength by a number of different chemistries. There isincreasing demand for structural and semi structural adhesives inautomotive applications. This is because joining with adhesives turnsout to be of lower cost and more efficient than welding or methods ofmechanical fastening. Adhesive bonding is essential for aluminium invehicles and also for composite or other hybrid constructions.

Structural and semi-structural adhesives are often used to provideadditional stiffness, fatigue and impact performance to body componentsand may be used in bonding stiffeners such as striker pinreinforcements; door handle reinforcements and other types ofreinforcement brackets in vehicles where higher strength, stiffness orfatigue properties are required than may be available from other methodsof joining such as welding, brazing or riveting for the same substratethickness.

Tape adhesives have been used for many years, but there are a number oflimitations to their use. First of all, it is difficult to apply a flattape to a curve which lies in the plane of the tape due to thedifficulty of accommodating compression and elongation in the sameplane. Therefore, curved, structures often need to be fitted with asequence of straight tape stripes gradually varying in their orientationon the surface to which they are applied.

Moreover, it is difficult to provide a tape which has good gap fillproperties. Adhesives used in the automotive industry should be able toabsorb tolerance variations between components during the joiningprocess and to allow movement, such as differential expansion, duringoven cycles. Often, joint gaps can vary in size by as much as 2 or 3 mm,and to bridge such a gap without reduction in performance or flutteringof the bonded parts, large quantities of liquid or paste type adhesiveare often required. In order to limit the cost of the adhesive which isused for gap filling, the respective adhesives are often highly filled.This leads to less than optimised weight and reduced vehicle efficiency.Environmentally less desirable polymer bases like PVC are alsofrequently used to minimise costs.

To provide gap-filling properties, expanding adhesives have beendeveloped, which incorporate micro-balloons or blowing agents which areheat activated, e.g. during a bake cycle. The problem with this approachis that the multidirectional and uneven expansion of such adhesives isdifficult to control. Thus, it is frequently difficult to determine howmuch adhesive needs to be applied and how the final orientation of theadhesive in the vehicle is going to affect expansion. In addition, theperformance of the adhesive, such as its strength or its attachment to asubstrate, may be impaired because of its foam construction. Foamingadhesives are often applied as castings or extrusions, kept in place byexpensive clips or secondary adhesives and fastenings.

Apart from disadvantages arising with respect to their handling,expanding adhesives which possess acceptable gap filling properties maynot always have sufficient bond strength to the substrates to which theyare applied. Finally, particularly in automotive applications, adhesivesmay need to be compatible with oily surfaces as substrates to avoid theneed for a pre-treatment of such surfaces.

A further problem with application of large masses of bulk adhesive incontact with closure panel skins as they are used e.g. in automotiveindustry is the contraction of the adhesive duringcuring/polymerisation. As a result, the outline of the adhesive may bediscernible as a ghost on the outer skin. Very small contractions can bevisible, and the read-through affects vehicle appearance and perceivedvalue.

Particularly for applications in the automotive industry, it is thusdesirable to provide an adhesive system which has the capability offilling variable gaps while retaining optimum physical properties of thecured/polymerised adhesive system. Moreover, the adhesive system shouldbe convenient to handle and to apply, and it should have a high bondstrength to a variety of surfaces as substrates, such as lubricatedsurfaces, without requiring their pre-treatment, such as a cleaning or acoating of the surfaces. Finally, the cured adhesive should becompatible with a number of chemical treatments to which the bondedproduct may be subjected, such as a phosphate wash carried out duringpaint line processing.

Adhesive tapes which are reinforced by fiber webs are disclosed in US2002/0182955 A1, which relates to a structural bonding tape combining anadhesive material and a fiber web, in DE 199 31 241 A1, which relates toan adhesive film for structural bonding and in JP-A-10-245534, whichrelates to a reinforced pressure sensitive adhesive tape.

The present invention addresses some of the difficulties and problemsdiscussed above by providing an elongated adhesive article comprising anelongated core, said core comprising a fibrous web and being providedwith a sheath comprising an adhesive composition, wherein the smallestdiameter of the core is 1 mm or more, preferably 2 mm or more or 3 mm ormore.

One advantage of the above system may be the combination of thecompressibility of the fibrous material comprised by the core which canbe used to occupy variable volumes with the bond strength achieved bythe adhesive composition comprised in the sheath. Use of a compressiblematerial instead of a liquid, foamable material or a paste mayfacilitate the handling and the application of the adhesive article tovarious geometries. Moreover, the compressible article may dispersestress and/or dampen vibrations. Via selection of different types ofadhesives and core materials, adhesive articles can be designed whichare suitable to cover a wide variety of applications.

Thus, in a first aspect, the invention relates to bonding andefficiently filling a gap between two or more substrates by means of anelongated adhesive article according to the invention. The adhesivearticle can be designed to stay in place by compression in a joint orsimply by the level of tack initially achieved. Application of such anarticle by hand is fast and convenient.

The adhesive article preferably provides compressibility, so that theadhesive sheath can bond to complex profiles without the need for anexpanding adhesive. Moreover, if the sheath comprises an activatableadhesive composition which may cure/polymerise or otherwise create abond upon activation, the rigidity of the resulting adhesive bond willlead to a reduction or a loss of the initial compressibility and thus tobonded structures of significant strength.

Furthermore, the core may comprise an initially compressible fibrous webthat has the capability of stiffening and/or crosslinking after theadhesive article has been fitted between substrates to be bonded. Thus,the compressive strength and the shear strength of the cured adhesivearticle as well as its dampening properties may be optimised.

The resulting joint construction may provide anti flutter and gapfilling properties and enhanced energy transfer between substrates bydistributing the load across the complete surface of the bond betweenthe substrates and minimizing stress concentrations typically found atedges of joints made with structural or semi-structural adhesive tapes.A further feature of the adhesive article is the ability to fillvariable gaps between substrates. Particularly reversibly compressibleadhesive articles tolerate movements of the substrate to which they arebonded, such as differential expansion during oven cycles. Furtherproperties may include the ability to occupy variable volumes whileproviding no squeeze-out of a liquid or paste filling system. Inaddition, the adhesive article according to the invention may offersound and vibration insulation.

An additional application for the technology of the present invention isthe manufacture of light-weight stiffening or dampening structuresobtainable by forming an array of the adhesive articles according to theinvention on a substrate or between two or more substrates.

FIG. 1 gives a schematic view of an adhesive article according to theinvention which is applied between two substrates

The adhesive article of the present invention has an elongated form dueto the presence of an elongated core on which a sheath is provided.Generally, the article has an aspect ratio, defined, as the length towidth ratio, with the length being the longitudinal axis and the widththe largest dimension of the cross-section, of at least 2, such as 4 ormore or 6 or more. In a particular embodiment, the adhesive article hasan aspect ratio of at least 10, such as 20 or more.

Examples for the shape of the cross-section of the adhesive articleperpendicular to its longitudinal axis are a circular shape, a polygonalshape such as a square, rectangle or a hexagon, or an elliptical shape.Often, the adhesive article takes the form of a cylinder or a polygonalcolumn, with the cylindrical shape being particularly preferred. If acore material is used which has an unfilled portion in its centre,tubular structures may also be obtained. As a rule, the adhesive articleis applied to a substrate with its longitudinal axis being parallel tothe surface of the substrate.

In order to ensure sufficient gap filling, the smallest diameter of thecore, which is generally perpendicular to the longitudinal axis, shouldbe 1 mm or more or 2 mm of more, such as 4 mm or more or 6 mm or more.Depending on the application of the adhesive article, cores with asmallest diameter perpendicular to the longitudinal axis of 10 mm ormore, such as 20 mm or more are suitable. For most purposes, cylindricalshapes of the adhesive article with a diameter of the core of 2 to 30 mmmay be used.

The length of the adhesive article is not particularly restricted, andit may be provided in a length of several meters, from which pieces ofsuitable length can be cut off upon use. To ensure convenient handling,the adhesive article could, for example, be produced in pre-cut pieceswith a length varying from 5 cm to 100 cm, which could also be furthercut to shorter lengths if needed.

The adhesive article according to the invention is preferablycompressible in at least one direction perpendicular to the longitudinalaxis of the elongated adhesive article.

The adhesive article according to the invention may be compressible byat least 30%, such as 50% or more, still preferably by at least 70% ormore or even 90% or more, under pressures which are typically appliedwhen bonding two substrates by means of an adhesive.

More preferably, the adhesive articles according to the invention can bereversibly compressed showing recoverable deformation of at least 10%,preferably at least 20% or even 30% along the thickness direction of thenon-woven fibrous web. In the context of the present invention, anadhesive article is considered to be reversibly compressible if, afterremoval of the load which is required to induce a reduction of thethickness of the article by the given percentage, the article returns toat least 90% of its original thickness.

Generally, the adhesive article should be compressible in any directionperpendicular to the longitudinal axis so that no specific orientationmust be chosen when applying the adhesive article to a substrate.However, it may be desirable for specific applications to use anadhesive article with anisotropic compressibility in differentdirections perpendicular to its longitudinal axis, which can then beapplied to the substrate in one or more preferred orientations.

When the adhesive article is compressed in one or more directionsperpendicular to its longitudinal axis, dimensional changes in otherdirections are acceptable and may even be desirable to increase the bondsurface, as illustrated in FIG. 1. FIG. 1 shows an adhesive articleaccording to the invention (10) having a circular cross-section inuncompressed form, which is positioned between two substrates (20). Whenpressure is applied perpendicular to the planes of the substrates (20)to achieve bonding, the adhesive article is compressed in response tothe pressure while its surface area in contact with the substratesincreases.

Preferably, the adhesive article and its core are reversiblycompressible in at least one or any direction perpendicular to thelongitudinal axis by at least 30%, such as 50% or more, still preferablyby at least 70% or more or even 95% or more. In the context of thepresent invention, an adhesive article is considered to be reversiblycompressible if, after removal of the load used to determine thecompressibility, the article returns to at least 90% of its originaldimension. Thus, the core may act as a spring providing pressure to theadhesive in the sheath which is in contact with the substrates to bebonded.

Furthermore, it is preferred that the adhesive article and its core areflexible along the longitudinal axis, i.e. it should be possible to bendthe adhesive article along its longitudinal axis. Thus, the adhesivearticle can be conveniently applied to a substrate in a variety ofgeometries, such as curves or corners.

A variety of fibrous webs can be used for the core in the context of thepresent invention, and examples include non-woven fibrous webs. Corescomprising a fibrous web are suitable for the purpose of the presentinvention, since they allow the properties of the adhesive article to beadapted to a wide variety of applications. Thus, by varying the densityof the web or the orientation of the fibers contained therein, thecompressibility of the web and its flexibility along the longitudinalaxis can be influenced. Moreover, by choosing suitable fibers and/oradditives, the properties of the web can be changed even after theadhesive article has been applied to a substrate.

Fiber properties could include flame retardant, antistatic, conductive,surface conductive, strain hardening, heat shrink, impact resistant,high or low temperature resistant, high or low tensile strength,depending on application requirements. Fiber appearance can also beoptimized to provide an interesting appearance by changing of colour orby provision of reflective properties which could be used to optimizeproduct appeal in packaging or to provide a particular identity or codefor customers to understand better product use for particularapplications. Fibers could also be designed to change colour dependanton strain or environmental exposure or temperature exposure in order todetermine the likely exposure history or progressive damage of anarticle on which the invention has been used.

Fibers can also be designed to be degradable (biologically, thermally orby exposure to other specific or general materials or mixtures) toenable improved recycling or dismantling characteristics of an articleon which the invention has been used. Depending on the application, itmay, for example, be desirable to remove the core from the adhesivearticle after it has been applied to at least one substrate.

Generally, the fibrous web has a basis weight ranging from 10 g/m² to 1kg/m² or to 500 g/m². Preferred are weights of 50 g/m² or above, such as70 g/m² or above or 80 g/m² or above. Particularly preferred are valuesof more than 100 g/m², such as 120 g/m² or above or 300 g/m² or above.

The type of fibers to be used in the fibrous web may be selectedaccording to the respective application. Typical examples are polyolefinfibers such as polyethylene, or polypropylene, polystyrene fibers,polyether fibers, polyester fibers such as polyethylene terephthalate(PET) or polybutaline terephthalate (PBT), vinyl polymer fibers such aspolyvinyl chloride and polyvinylidene fluoride, poly amides such aspolycaprolactame, polyurethanes, nylon fibers, polyaramide fibers (suchas Kevlar® fibers). Alternatively or in addition, the fibrous web maycontain metal fibers, metal coated polymeric fibers, polymeric fiberscoated with other conductive materials or carbon fibers. Preferredexamples are PET fibers and polyaramide fibers. Also, blends of two ormore of the above types of fibers may be used to provide fibrous webs inthe adhesive article of the invention.

Generally, the fibers of the fibrous web have a diameter of at least 1denier (den). Preferred are values greater than 5 den, such as 10 den orabove or 15 den or above. Preferably, the fibers should have a diameterof not more than 100 den, such as 50 den or less.

Preferably, the fibers defined above are present in a fibrous web asstaple fibers. In addition, the web may contain thermal bond fiberswhich are used to give further strength to the fibrous web. Such thermalbond fibers are known in the art, and typical examples of such fibersare bicomponent fibers (also known as Bico fibers). Generally, thesethermal bond fibers have a fiber diameter typically in the range of 1 to20 den. The proportion of staple fibers to thermal bond fibers can be inthe range of 5 to 95%, more typically in the range of 10 to 50%, forexample around 30%.

Fibrous webs which have the desired fiber structure may be obtained bymethods known in the art, such as carding and cross-lapping (CCL) or byRando-webbing.

In the carding and cross-lapping process, pre-opened fibers are fed intoa card, which opens the fibers by combing them between roles. The fibersare then laid down onto a belt; fibers form a lengthwise orientedpre-web. The web is then passed through a cross-lapper and theone-dimensional, machine direction oriented, pre-web is layered to givea controlled thickness and weight. This process arranges the fiberspartially in the cross web direction. The web is then normally passedthrough a thermal bond oven to heat bond the fibres together and givethe CCL web integrity. The Rando Webber is a commercial piece ofprocessing equipment that is commonly used to process short fibers intononwoven webs. Pre-opened and blended fibers are fed into the feedsection of the Rando. The unit then typically will further tumble thefibers as it selectively feeds the Lickerin, from which the fibers arethen blown onto a vacuum collection surface and are conveyed away fromthe machine for post web processing (thermal bonding).

Fibrous webs which are also suitable as core materials in the presentinvention can be obtained by a technique known as vertical lapping orvertical cross lapping (VCL). Vertical cross lapping yields a corrugatedfibrous web by vertically folding a carded fibrous web. The fibers whichare initially oriented in the y-direction in the carded web used as astarting product in the VCL process, can be oriented towards thez-direction (or the thickness direction) of the fibrous web resultingfrom the process. Thermal bond fibers help to stabilize the corrugatedstructure of the vertically cross lapped web. These thermal bond fibersare typically activated after the lapping process, e.g. by passing theweb through a thermal bond oven. Suitable techniques of vertical crosslapping are established on the art, and the equipment often used toobtain a VCL web is available under the tradename “Struto” or“Wavemaker”. Such techniques are also disclosed in U.S. Pat. No.6,602,581 and literature cited therein. If the core of the adhesivearticle comprises such a vertically cross lapped web, the y-axis of theweb should be oriented in the longitudinal axis of the core. Thus, anadhesive article can be obtained which shows a highly reversiblecompressibility perpendicular to its longitudinal axis and can be easilybent along this longitudinal axis.

By using cores comprising a fibrous web, the adhesive article of thepresent invention can be specifically designed to turn from an initiallysoft, compressible state to a more stiff or rigid state which provides ahigh degree of strength, impact and compression resistance after itsapplication to the desired position.

Such a stiffening of the fibrous web can be achieved by a variety ofmethods. For example, the fibrous web may comprise a composition that,upon activation to heat, actinic radiation or electron beam irradiation,causes fixation of the fibers to another or fixation of the fibers in amatrix so as to cause reduced compressibility of the fibrous web or aloss of compressibility. The composition may be included into the webfor example in the form of a powder, or in the form of fibers such asthermal bond fibers.

In one embodiment, the fibrous web can be loaded with a powder of athermosettable or thermoplastic composition which, upon activation bondthe fiber structure together or flow into each other to form acontinuous stable matrix around the fibers. Suitable compositions arethose referred to below in the context of the activatable adhesivecomposition. Generally, it is sufficient to use these materials in anamount of 5 weight % or more, based on the weight of the unloadedfibrous web.

In one embodiment, thermal bond fibers, such as those defined above mayalso be used to increase the stiffness of the fibrous web after theadhesive article of the invention has been applied to a substrate. Ifdesired, thermal bond fibers having different melting points may beincluded into the fibrous web. Thus, one type of thermal bond fibers(Low melting temperature) may serve for stabilizing the web aftervertical lapping, and another type (High melting temperature) forfurther stiffening the web at the site of use of the adhesive article.

In the adhesive article according to the invention, the core is providedwith a sheath comprising an adhesive composition. The sheath may form aseparate layer which is in contact with the surface of the core, or thecore may be partially embedded in the material constituting the sheath.On the other hand, the core should generally not be fully impregnatedwith the material of the sheath, so as to maintain its compressibility.Typically, the sheath extends into the core by not more than 50% or 30%of the diameter of the core in perpendicular direction to itslongitudinal axis, preferably not more than 20% or 10%, and particularlypreferably or not more than 2%.

As a rule, the sheath has a thickness of 0.05 mm to 0.5 mm, preferably0.1 mm to 0.3 mm.

Examples for the adhesive composition comprised in the sheath are anactivatable adhesive composition or a pressure sensitive adhesivecomposition or combinations of both. Often, these adhesive compositionshave a sufficient flexibility in the non-activated state so as not tointerfere with the compressibility and/or flexibility of the core whenthe adhesive article is applied.

One possible joining process using the adhesive article of the inventionwould consist of application of the adhesive article to one surface andthen bringing a second surface in contact with it while compressing thejoint to the correct height. It is generally desirable but not essentialfor the activatable adhesive to have initial tack to the bonding surfacein order to enable components to achieve good initial positioning orhandling strength. It is also possible to apply the adhesive article toa single surface, e.g. for stiffening or other reasons. In this case itwould be desirable but not essential for the activatable adhesive tohave a level of tack to enable easy positioning without slippage. Thereare alternative applications where tack may not be preferred in theactivatable adhesive, such as in applications where a certain amount ofslippage may be required. It is also possible to provide adhesivearticles in which the activatable adhesive used in the sheath would haveonly one tacky surface or for specific areas of tack to be usedintermittently in the otherwise non-tacky but activatable adhesive.

The adhesive article of the present invention may be provided with bothinitial adhesive properties and a high bond strength by including intothe sheath an activatable adhesive in combination with a pressuresensitive adhesive. For example, the activatable adhesive compositionand the pressure sensitive adhesive may be present in the sheath in theform of a multitude of separate domains, each of said domains defining apart of the surface of the adhesive layer. Alternatively, the pressuresensitive adhesive composition may form a thin layer on the activatableadhesive composition, as disclosed in U.S. Pat. No. 5,593,759, or thepressure sensitive and the activatable adhesive composition may bedistributed in the sheath as disclosed in U.S. Pat. No. 5,585,178.

The term ‘activatable adhesive composition’, as it is used in thecontext of the present invention, refers to an adhesive composition thatrequires activation to form a bond, in particular a permanent bond. By‘activation’ is meant that the adhesive composition is exposed to heator is irradiated with, for example UV, visible light or e-beam to causethe bond to form. Generally, a structural bond should be formed that hasa shear strength of at least 2 MPa, preferably at least 6.9 MPa, morepreferably at least 15 MPa measured according to ASTM D-1002-94. Thestructural bond preferably has a T peel strength of at least 50N/25 mm,more preferably at least 100N/25 mm when tested according to the methodsoutlined under ASTM D1876. The activatable adhesive composition may ormay not have pressure sensitive adhesive properties, and, when present,these may not be retained through a typical activation cycle used informing a permanent or structural bond. Exemplary activatable adhesivecomposition having pressure sensitive properties which may also be usedfor the adhesive article of the present invention are disclosed in U.S.Pat. No. 5,086,088 and in WO 92/20754.

Upon activation, the activatable adhesive composition may cross-link orcure, i.e. a so-called thermosetting adhesive, or the adhesivecomposition may melt, thereby wetting out the surface and forming a bondupon cooling. Still further, the activatable adhesive composition may becomprised of a so-called hybrid material as defined below.

The term “thermosetting” as used herein refers to a material, whichundergoes a curing reaction that results in a chemical change uponbonding and an increase in the hardness of the material. The term“thermoset” as used herein refers to a thermosetting material, which hasbeen cured. A thermosetting material may generally be bonded byapplication of heat, actinic radiation such as UV, visible, or infrared,or microwave or X-ray energy.

The term “thermoplastic” as used herein refers to a material whichundergoes a physical change upon the application of heat, i.e., thematerial flows upon bonding and returns to its initial non-flowing stateupon cooling. A thermoplastic material is typically bonded byapplication of heat.

The term “hybrid material” refers to a material which is a combinationof at least two components, wherein the at least two components arecompatible in the melt phase (the melt phase is where the combination ofthe at least two components is a liquid), the at least two componentsform an interpenetrating polymer network or semi-interpenetratingpolymer network, and at least one component becomes infusible (i.e., thecomponent cannot be dissolved or melted) after application of heat or byother means of curing such as application of light. A hybrid materialwill be described in more detail below. A hybrid material may generallybe bonded by application of heat, actinic radiation such as UV, visible,or infrared, or microwave or X-ray energy.

A hybrid material is, for the purpose of the present invention, mutuallyexclusive of the classes of thermosetting and thermoplastic materialsdefined herein. In other words, thermosetting materials and any optionaladditives or thermoplastic materials and any optional additives will beconsidered non-hybrid materials if they do not meet the definition ofhybrid material as defined herein.

Exemplary activatable compositions for use in the present invention arethose referred to as adhesive material in US 2002/0182955 A1, thecurable compositions of U.S. Pat. No. 6,057,382 or those listed in thefollowing

Suitable thermosetting materials include epoxides, urethanes, cyanateesters, bismaleimides, phenolics, including nitrile phenolics, and anycombinations thereof.

Suitable epoxides include those containing at least two 1,2-cyclicethers. Such compounds can be saturated or unsaturated, aliphatic,aromatic or heterocyclic, or can comprise combinations thereof. Suitableepoxides may be solid or liquid at room temperature.

Compounds containing at least two epoxide groups (i.e., polyepoxides)are preferred. A combination of epoxide compounds may be employed, andan epoxide having a functionality of less than two may be used in acombination so long as the overall epoxide functionality of the mixtureis at least two. The polymeric epoxides include linear polymers havingterminal epoxy groups (e.g., a diglycidyl ether of a polyoxyalkyleneglycol), polymers having skeletal oxirane units (e.g., polybutadienepolyepoxide), and polymers having pendent epoxy groups (e.g., a glycidylmethacrylate polymer or copolymer). It is also within the scope of thisinvention to use a material with functionality in addition to epoxidefunctionality but which is essentially unreactive with the epoxidefunctionality, for example, a material containing both epoxide andacrylic functionality.

A wide variety of commercial epoxides are available and listed in“Handbook of Epoxy Resins” by Lee and Neville, McGraw Hill Book Company,New York (1967) and in “Epoxy Resin Technology” by P. F. Bruins, JohnWiley & Sons, New York (1968), and in “Epoxy Resins: Chemistry andTechnology, 2nd Edition” by C. A. May, Ed., Marcel Dekker, Inc. New York(1988). Aromatic polyepoxides (i.e., compounds containing at least onearomatic ring structure, e.g., a benzene ring, and at least two epoxidegroups) that can be used in the present invention include thepolyglycidyl ethers of polyhydric phenols, such as Bisphenol A- orBisphenol-F type resins and their derivatives, aromatic polyglycidylamines (e.g., polyglycidyl amines of benzenamines, benzene diamines,naphthylenamines, or naphthylene diamines), polyglycidyl ethers ofphenol formaldehyde resole or novolak resins; resorcinol diglycidylether; polyglycidyl derivatives of fluorene-type resins; and glycidylesters of aromatic carboxylic acids, e.g., phthalic acid diglycidylester, isophthalic acid diglycidyl ester, trimellitic acid triglycidylester, and pyromellitic acid tetraglycidyl ester, and mixtures thereof.Preferred aromatic polyepoxides are the polyglycidyl ethers ofpolyhydric phenols, such as the series of diglycidyl ethers ofBisphenol-A commercially available from Shell Chemical Inc., Houston,Tex., for example, under the trade designations “EPON 828” and “EPON1001 F” and the series of diglycidyl ethers of Bisphenol-A and BisphenolF and their blends commercially available from Shell Chemical Inc., forexample, under the trade designations “Epikote 232” and “Epikote 1001”available from Shell Chemical Inc., Pemis, The Netherlands. Other usefulcommercially available aromatic epoxides include the “DER” series ofBisphenol epoxides, and “DEN” series of epoxy novolak resins availablefrom Dow Chemical, Midland, Mich., diglycidyl ether of fluoreneBisphenol, available from Shell Chemical Inc., Houston, Tex., under thetrade designation “EPON HPT Resin 1079”, a triglycidyl derivative ofp-aminophenol commercially available from Ciba Performance Polymers,Brewster, N.Y. under the trade designation “MY 0500”, a tetraglycidylderivative of methylene dianiline commercially available from CibaPerformance Polymers, Brewster, N.Y. under the trade designation “MY720”. Flame retardant epoxides may also be used, for example, the flameretardant brominated Bisphenol-A diglycidyl ether commercially availablefrom Dow Chemical, Midland, Mich., under the trade designation “DER580”. The term “derivative” as used herein with reference tothermosetting materials refers to a base molecule with additionalsubstituents that do not interfere with the thermosetting bonding of thebase molecule.

Representative aliphatic cyclic polyepoxides (i.e., cyclic compoundscontaining one or more saturated carbocyclic rings and at least twoepoxide groups, also known as alicyclic compounds) useful in the presentinvention include the series of alicyclic epoxides commerciallyavailable from Union Carbide Corp., Danbury, Conn., under the tradedesignation “ERL”, such as vinyl cyclohexene dioxide (“ERL-4206”),3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (“ERL-4221”),3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate (“ERL-4201”),bis(3,4-epoxy-6-methylcycylohexylmethyl)adipate (“ERL-4289”), anddipentenedioxide (“ERL-4269”).

Representative aliphatic polyepoxides (i.e., compounds containing nocarbocyclic rings and at least two epoxide groups) include1,4-bis(2,3-epoxypropoxy)butane, polyglycidyl ethers of aliphaticpolyols such as glycerol, polypropylene glycol, 1,4-butanediol, and thelike, the diglycidyl ester of linoleic acid dimer, epoxidizedpolybutadiene (for example, those available under the trade designation“OXKON 2001” from FMC Corp., Philadelphia, Pa. or “Poly bd” from ElfAtochcm, Philadelphia, Pa.), epoxidized aliphatic polyurethanes, andepoxy silicones, e.g., dimethylsiloxanes having cycloaliphatic epoxideor glycidyl ether groups.

Examples of suitable epoxide-based bondable layers that are commerciallyavailable in film form include those available from Minnesota Mining andManufacturing Company (“3M”), St. Paul, Minn., under the tradedesignation “3M Scotch-Weld Structural Adhesive Film” including thosehaving the following “AF” designations: “AF 42”, “AF 111”, “AF 126-2”,“AF 163-2”, “AF 3109-2”, “AF 191”, “AF 2635”, “AF 3002”, “AF 3024”, and“AF 3030FST”.

In one embodiment, a thermosetting activatable adhesive comprises afusible epoxide prepolymer (which can melt and flow unlike a B stageresin) which is a solid at room temperature and, more preferably,further comprises a second epoxide component which may be solid orliquid at room temperature. Suitable solid fusible epoxide prepolymersinclude those described above which are a solid at room temperature.

An exemplary activatable adhesive composition may comprise a solidfusible epoxide prepolymer comprising a diglycidyl ether of Bisphenol Aalone or in combination with a diglycidyl ether of Bisphenol A orBisphenol F or a blend thereof. The activatable adhesive composition isa solid at room temperature after the addition of any optionalcomponents, more preferably the epoxide material (comprising single ormultiple epoxides) is a solid at room temperature.

The term “urethane materials” as used herein applies to polymers madefrom the reaction product of a compound containing at least twoisocyanate groups (—N═C═O), referred to herein as “isocyanates”, and acompound containing at least two active-hydrogen containing groups.Examples of active-hydrogen containing groups include primary alcohols,secondary alcohols, phenols and water; and primary and secondary amines(which react with the isocyanate to form a urea linkage). A wide varietyof isocyanate-terminated materials and appropriate co-reactants are wellknown, and many are commercially available (see for example, GunterOertel, “Polyurethane Handbook”, Hanser Publishers, Munich (1985)).

In order to prepare storage-stable bondable layers based on urethanematerials it is preferable to use either an isocyanate or an activehydrogen-containing compound that is blocked. The term “blocked” as usedherein refers to a compound that has been reacted with a second compound(i.e. “blocking group”) such that its reactive functionality is notavailable until such time as the blocking group is removed, for exampleby heating, or by further reaction, such as with water. Examples ofblocked isocyanates include those that have been co-reacted with phenol,methyl ethyl ketoxime, and epsilon-caprolactam. Examples of blockedactive-hydrogen containing compounds include aldehyde or ketone blockedamines (known as ketimines); aldehyde blocked aminoalcohol (known asoxazolidines); and amines that have been complexed with a salt such assodium chloride.

When blocked isocyanates are used, examples of suitable co-reactantsinclude polyether polyols such as poly(oxypropylene) glycols, ethyleneoxide capped poly(oxypropylene) glycols, and poly(oxytetramethylene)glycols; diamino poly(oxypropylene) glycols; aromatic amine terminatedpoly(propylene ether) glycols; styrene-acrylonitrile graft polyols;poly(oxyethylene) polyols; polyester polyols such as polyglycoladipates, polyethylene terephtrialate polyols, and polycaprolactonepolyols; polybutadiene polyols, hydrogenated polybutadiene polyols,polythioether polyols, silicone carbinol polyols, polybutylene oxidepolyols, acrylic polyols, carboxy-functional polypropylene oxidepolyols, carboxy functional polyester polyols; and aromaticamine-terminated poly(tetrahydrofuran). Suitable urethane resins includeblocked urethanes such as that available under the trade designation“Adeka Resin QR-9276” from Asahi Denka Kogyo K. K. Tokyo, Japan, andurethane modified epoxides such as that available under the tradedesignation “Rutapox VE 2306” from Rutgers Bakelite GmbH, Duisburg,Germany.

Suitable cyanate ester materials (monomers and oligomers) are thosehaving two or more —OCN functional groups, including those described inU.S. Pat. No. 5,143,785, incorporated herein by reference. Examples ofsuitable cyanate ester compounds include the following: 1,3- and1,4-dicyanatobenzene; 2-tert-butyl-1,4-dicyanatobenzene;2,4-dimethyl-1,3-dicyanatobenzene;2,5-di-tert-butyl-1,4-dicyanatobenzene;tetramethyl-1,4-dicyanatobenzene, 4-chloro-1,3-dicyanatobenzene;1,3,5-tricyanatobenzene; 2,2,- or 4,4,-dicyanatobiphenyl;3,3′,5,5′-tetramethyl-4,4′,-dicyanatobiphenyl; 1,3-, 1,4-, 1,5-, 1,6-,1,8-, 2,6-, or 2,7-dicyanatonaphthalene; 1,3,6-tricyanatonaphthalene;bis(4-cyanatophenyl)methane; bis(3-chloro-4-cyanatophenyl)methane;bis(3,5-dimethyl-4-cyanatophenyl)methane;1,1-bis(4-cyanatophenyl)ethane; 2,2-bis(4-cyanatophenyl)propane;2,2-bis(3,5-dibromo-4-cyanatophenyl)propane;2,2-bis(4-cyanatophenyl)-1,1,1,3,3,3-hexafluoropropane;bis(4-cyanatophenyl)ether; bis(4-cyanatophenoxyphenoxy)benzene;bis(4-cyanatophenyl)ketone; bis(4-cyanatophenyl)thioether;bis(4-cyanatophenyl)sulfone; tris(4-cyanatophenyl)phosphite; andtris(4-cyanatophenyl)phosphate. Polycyanate compounds obtained byreacting a phenol-formaldehyde precondensate with a halogenated cyanideare also suitable.

Other suitable materials include cyanic acid esters derived fromphenolic resins as described in U.S. Pat. No. 3,962,184, cyanatednovolac resins derived from, novolac resins as described in U.S. Pat.No. 4,022,755, cyanated bisphenol-type polycarbonate oligomers derivedfrom bisphenol-type polycarbonate oligomers as described in U.S. Pat.No. 4,026,913, cyanato-terminated polyarylene ethers as described inU.S. Pat. No. 3,595,900, dicyanate esters free of ortho hydrogen atomsas described in U.S. Pat. No. 4,740,584, mixtures of di- and tricyanatesas described in U.S. Pat. No. 4,709,008, polyaromatic cyanatescontaining polycyclic aliphatics as described in U.S. Pat. No.4,528,366, ffluorocarbon cyanates as described in U.S. Pat. No.3,733,349, and other cyanate compositions as described in U.S. Pat. Nos.4,195,132 and 4,116,946, all of which are incorporated herein byreference. An exemplary commercially available material is a cyanateester available from Ciba Performance Polymers, Brewster, N.Y. under thetrade designation “Quatrex 7187”.

Suitable phenolic resins are generally described in Encyclopedia ofPolymer Science and Engineering, Volume 11, John Wiley & Sons, Inc. (NewYork, 1988), pp. 45-92. Phenolic-based resins are generally described inAlphonsus V. Pocius, Adhesion and Adhesives Technology: An Introduction,Hanser Publishers (New York, 1997), pp. 185-188. Preferred phenolicresins that can be used to impregnate a sheet which is suitable toprepare hot press laminated products from wood veneers are discussed inU.S. Pat. No. 1,960,176, incorporated herein by reference. Suitablephenolic materials are those made as the reaction product of phenols andformaldehydes, including resole phenolics and novolac phenolics.Examples of phenols include phenol, resorcinol, para-substituted phenol,cresol, and the reaction product of bisphenol A and the monoglycidylether of bisphenol A. Exemplary phenolic-based bondable layers includetissue paper impregnated with a thermosetting phenolic resin at a ratioof approximately two parts resin to one part tissue paper commerciallyavailable under the trade designation “Phenolic Glue Film” from DynoOverlays Inc., High Point, N.C.

Resole phenolic resins are characterized by being alkaline catalyzed andhaving a molar ratio of formaldehyde to phenol of greater than or equalto 1:1. Typically, the ratio of formaldehyde to phenol is within a rangeof about 1:1 to about 3:1. Examples of suitable alkaline catalysts forpreparing resole phenolic resins include sodium hydroxide, potassiumhydroxide, organic amines, or sodium carbonate.

Novolac phenolic resins are characterized by being acid, catalyzed andhaving a molar ratio of formaldehyde to phenol of less than 1:1.Typically, the ratio of formaldehyde to phenol is within a range ofabout 0.4:1 to about 0.9:1. Examples of the acid catalysts used toprepare novolac phenolic resins include sulfuric, hydrochloric,phosphoric, oxalic, or p-toluenesulfonic acids. Although novolacphenolic resins are typically considered to be thermoplastic resinsrather than thermosetting resins, they can react with other chemicals(e.g., hexamethylenetetraamine) to form a thermoset resin.

Examples of useful commercially available resole or novolac phenolicresins include “Varcum” from BTL Specialty Resins Corporation, BlueIsland, DLL; “Arofene” from Ashland Chemical Company, Columbus, Ohio;“Bakelite” from Union Carbide, Danbury, Conn.; and “Resinox” fromMonsanto Chemical Company, St. Louis, Mo.

Suitable nitrile phenolic materials include those made by includingbutadiene-nitrile elastomers in novolac phenolic resin-based materials.Examples of suitable nitrile phenolic based bondable layers that arecommercially available in film form include those available fromMinnesota Mining and Manufacturing Company (“3M”), St. Paul, Minn.,under the trade designation “3M Scotch-Weld Structural Adhesive Film”and having the following “AF” designations: “AF 10”, “AF 30”, “AF 31”and “AF 32”.

Examples of suitable bismaleimide materials, also knoAvn asN,N′-bismaleimide monomers and prepolymers, include theN,N′-bismaleimides of 1,2-etraanediamine, 1,6-hexanediamine,trimethyl-1,6-hexanediamine, 1,4-benzenediamine,4,4′-methylene-bis(benzenamine), 2-methyl-1,4-benzenediamine,3,3′-methylene-bis(benzenamine), 3,3′-sulfonyl-bis(benzenamine),4,4′-sulfonyl-bis(benzcnamine), 3,3′-oxy-bis(benzenamine),4,4′-oxy-bis(benzenamine), 4,4′-methylene-bis(cyclohexanamine),1,3-benzenedimethanamine, 1,4-benzenedimethanamine, and4,4′-cyclohexane-bis(benzenamine) and mixtures thereof; OtherN,N′-bis-maleimides and their process of preparation are described inU.S. Pat. Nos. 3,562,223; 3,627,780; 3,839,358; and 4,468,497, all ofwhich are incorporated herein by reference. Representative examples ofcommercially available bismaleimide materials include the series ofmaterials available from Shell Chemical, Houston, Tex. under the tradedesignation “COMPIMIDE” such as 4,4′-bismaleimidodiphenyl methane(“COMPTMIDE Resin MDAB”), and 2,4′-bismaleimidotoluene (“COMPIMIDE ResinTDAB”), and from Dexter/Quantum, San Diego, Calif., under the tradedesignation “Q-Bond”.

A thermosetting bondable layer preferably comprises a thermosettingmaterial and a curative or curatives. The term “curative” is usedbroadly to include not only those materials that are conventionallyregarded as curatives but also those materials that catalyze oraccelerate the reaction of the curable material as well as thosematerials that may act as both curative and catalyst or accelerator. Itis also possible to use two or more curatives in combination.

Preferred heat activated curatives for use in the present inventionexhibit latent thermal reactivity; that is, they react primarily athigher temperatures (preferably at a temperature of at least 80° C.), orreact at lower temperatures only after an activation step such asexposure to actinic radiation. This allows the adhesive composition tobe readily mixed and coated at room temperature (about 23±3° C.) or withgentle warming without activating the curative (i.e., at a temperaturethat is less than the reaction temperature for the curative). Oneskilled in the art would readily understand which curatives areappropriate for each class of thermosetting materials.

Suitable curatives for epoxide polymerization include polybasic acidsand their anhydrides; nitrogen-containing curatives; chloro-, bromo-,and fluoro-containing Lewis acids of aluminum, boron, antimony, andtitanium; photochemically activated generators of protic or Lewis acids;and phenolic materials as described above. Exemplary polybasic acids andtheir anhydrides include di-, tri-, and higher carboxylic acids such asoxalic acid, phthalic acid, terephthalic acid, succinic acid, alkylsubstituted succinic acids, tartaric acid, phthalic anhydride, succinicanhydride, malic anhydride, nadic anhydride, pyromellitic anhydride; andpolymerized acids, for example, those containing at least 10 carbonatoms, such as dodecendioic acid, 10,12-eicosadiendioic acid, and thelike.

Nitrogen-containing curatives include, for example, dicyandiamide,imidazoles (e.g. hexakis(imidazole) nickel phthalate), imidazolates,dihydrazides (e.g. adipic dihydrazide and isophthalic dihydrazide),ureas, and melamines, as well as encapsulated aliphatic amines (e.g.,diethylenetriamine, triethylenetetraamine, cyclohexylamine,triethanolamine, piperidine, tetramethylpiperamine,N,N-dibutyl-1,3-propane diamine, N,N-diethyl-1,3-propane diamine,1,2-diamino-2-methyl-propane, 2,3-diamino-2-methyl-butane,2,3-diamino-2-methyl-pentane, 2,4-diamino-2,6-dimethyl-octane,dibutylamine, and dioctylamine). The term “encapsulated” as used hereinmeans that the amine is surrounded by a material that prevents it fromacting as a curative until the application of heat. Polymer bound aminesor imidazoles may also be used. Pyridine, benzylamine,benzyldimethylamine, and diethylaniline are also useful as heatactivated curatives.

Examples of nitrogen-containing curatives include those commerciallyavailable from Air Products, Allentown, Pa., under the tradedesignations, “Amicure CG-1200”, “AMICURE CG-1400”, “Ancamine 2337”,“Ancamine 2441”, “Ancamine 2014”; and those from Asahi Denka Kogyo K. K.Tokyo, Japan, under the trade designations “Ancamine 4338S” and“Ancamine 4339S”; those from CVC Specialty Chemicals, Mapleshade, N.J.,under the trade designations “Omicure U-52” and “Omicure U-410” as wellas the other materials in the “Omicure” series; those fromLandec, MenloPark, Calif., under the trade designations “Intellimer 7001”,“Intellimer 7002”, “Intellimer 7004”, and “Intellimer 7024”; those fromShikoku Fine Chemicals, Japan, and sold by Air Products, as the seriesof materials available under the trade designation “Curezol”; and thosefrom Ajinomoto Company Inc., Teaneck, N.J., as the series of materialsavailable under the trade designation “Ajicure”.

Exemplary chloro-, bromo-, and fluoro-containing Lewis acids ofaluminum, boron, antimony, and titanium include aluminum trichloride,aluminum tribromide, boron trifluoride, antimony pentafluoride, titaniumtetrafluoride, and the like. Preferably, these Lewis acids may beblocked to increase the latency of the thermosetting material.Representative blocked Lewis acids include BF₃-monoethylamine, and theadducts of HSbF₅ X, in which X is halogen, —OH, or —OR¹ in which R¹ isthe residue of an aliphatic or aromatic alcohol, aniline, or aderivative thereof, as described in U.S. Pat. No. 4,503,211,incorporated herein by reference.

Suitable photochemically activated curatives for epoxide polymerizationinclude cationic photocatalysts that generate an acid to catalyzepolymerization. It should be understood that the term “acid” can includeeither protic or Lewis acids. These cationic photocatalysts can includea metallocene salt having an onium cation and a halogen containingcomplex anion of a metal or metalloid. Other useful cationicphotocatalysts include a metallocene salt having an organometalliccomplex cation and a halogen-containing complex anion of a metal ormetalloid which are further described in U.S. Pat. No. 4,751,138 (e.g.,column 6, line 65 to column 9, line 45). Other examples of usefulphotocatalysts include organometallic salts and onium salts, forexample, those described in U.S. Pat. No. 4,985,340 (e.g., col. 4, line65 to col. 14, line 50) and in European Patent Applications 306,161 and306,162. Still other cationic photocatalysts include an ionic salt of anorganometallic complex in which the metal is selected from the elementsof Periodic Group IVB, V13, VIB, V1113 and VBTB which is described inEuropean Patent Application 109,581. A suitable photochemicallyactivated curative is a curative commercially available from Ciba-Geigy,Hawthorne, N.Y. under the trade designation “Irgacure 261”.

Suitable curatives for urethane materials include thenitrogen-containing curatives as described for use with epoxides (whichcan react with a blocked isocyanate group after the deblocking reactionto give a urea) as well as, for example, materials containing hydroxyl(e.g., phenols) or thiol functionality that can react with the deblockedisocyanate. Photochemically activated generators of protic or Lewisacids can be used to enhance these reactions.

Suitable curatives for cyanate ester materials include thenitrogen-containing curatives as described for use with epoxides as wellas curatives that may be thermally or photochemically activated.Examples of such curatives include organometallic compounds containing acyclopentadienyl group (C₅H₅) and derivatives of a cyclopentadienylgroup. Suitable curatives include cyclopentadienyl iron dicarbonyl dimer([C₅H₅Fe(CO)₂]₂), pentamethylcyclopentadienyl iron dicarbonyl dimer([C₅(CH₃)₅Fe(CO)₂]₂), methylcyclopentadienyl manganese tricarbonyl(C₅H₄, (CH₃)Mn(CO)₃), cyclopentadienyl manganese tricarbonyl(C₅H₅Mn(CO)₃), all of which are available from Strem Chemical Company,Newburyport, Mass. Other suitable curatives include thehexafluorophosphate salt of the cyclopentadienyl iron mesitylene cation(C₅H₅ (mesitylene)Fe⁺PF₆ ⁻), and the trifluoromethanesulfonate salt ofthe cyclopentadienyl iron mesitylene cation (C₅H₅ (mesitylene)Fe⁺(CF₃SO₃ ⁻)), both of which may be prepared by methods described in U.S.Pat. No. 4,868,288 which is incorporated herein by reference.

Suitable curatives for phenolic materials and for nitrile phenolicmaterials include hexamethylene tetraamine (a latent source offormaldehyde) as well as combinations of organic acids (e.g. phosphoricacid, para toluene sulfonic acid, and salicylic acid) and metallicoxides (e.g. zinc oxide and magnesium oxide).

Suitable curatives for bismaleimide materials include the nitrogencontaining curatives as described for use with epoxides as well aslatent sources of allyl phenol.

In an alternative embodiment, the activatable adhesive composition maybe based on a thermoplastic material. Suitable thermoplastic materialsinclude, for example, polyesters, ethylene vinyl acetate (EVA),polyurethanes, polyamides, polyolefins, and derivatives thereof. Theterm “derivative” as used herein with reference to thermoplasticmaterials refers to a base molecule with additional substituents thatare not reactive towards a crosslinking or polymerization reaction.Thermoplastic materials, by nature, typically do not require curatives.

A hybrid material, as a further embodiment of the activatable adhesivecomposition, is a combination of at least two components wherein the atleast two components are compatible in the melt phase (the melt phase iswhere the combination, of the at least two components is a liquid), theat least two components form an interpenetrating polymer network orsemi-interpenetrating polymer network, and at least one componentbecomes infusible (i.e., the component cannot be dissolved or melted)after application of heat or by other means of curing such asapplication of light. A first component is a crosslinkable material anda second component is (a) a thermoplastic material, (b) monomers,oligomers, or polymers (and any required curative) which can form athermoplastic material, (c) a thermosetting material, i.e., monomers,oligomers, or prepolymers (and any required curative) which can form athermosetting material. The second component is chosen so that it is notreactive with the first component. It may be desirable, however, to adda third component which may be reactive with either or both of thecrosslinkable material and second component for the purpose of, forexample, increasing the cohesive strength of the bonded hybrid material.

Suitable first components include thermosetting materials, for example,the thermosetting materials described above, as well as crosslinkableelastomers such as acrylics and urethanes as described above.

Suitable thermoplastic second components include those thermoplasticmaterials described above. Suitable thermoplastics which can be formedin situ, i.e., with monomers, oligomers, or polymers (and any requiredcurative) which can form a thermoplastic material without undergoing anysignificant crosslinking reaction would be readily apparent to oneskilled in the art. Exemplary hybrid materials incorporating a secondcomponent (a) are described, for example, in PCT/EP98/06323; U.S. Pat.No. 5,709,948, and U.S. Sen No. 09/070,971, all of which areincorporated herein by reference. Exemplary hybrid materialsincorporating a second component (b) are described, for example, in U.S.Pat. No. 5,086,088, which is incorporated herein by reference. Example 1of U.S. Pat. No. 5,086,088 illustrates an example of a thermoplasticmaterial formed in situ.

Suitable thermosetting second components include those thermosettingmaterials described above. Exemplary hybrid materials incorporating asecond component (c) are described, for example, in U.S. Pat. No.5,494,981, which are incorporated herein by reference.

A particularly preferred activatable adhesive composition is an epoxybased composition as disclosed in U.S. Pat. No. 6,506,494. Thus, in thisembodiment, the activatable adhesive composition comprises:

A) an epoxy resin capable of being cured to a cured epoxy resin whenexposed to an activated latent curative system;B) a latent curative system in an amount sufficient to cure said epoxyresin, comprising (a) at least one epoxy resin miscible first curativecomprising a latent hardener, selected from dicyandiamide and itsderivatives, contained substantially as a core within a multiplicity ofambient-temperature-stable, impermeable microcapsules having capsulewalls comprised of a thermoplastic polymeric material and (b) at leastone epoxy resin latent second curative comprising a latent acceleratorwhich is a metal imidazolate compound. The metal imidazolate may be acompound of the formula:

ML_(m)

wherein M is a metal selected from the group of Ag(I), Cu(I), Cu(II),Cd(II), Zn(II), Hg(II), Ni(II) and Co(II),L is an imidazolate of the formula:

wherein R¹, R², and R³ are selected from a hydrogen atom, an alkylradical typically having 1 to 10, preferably 1 to 6 carbon atoms or anaryl radical typically having 6 to 18, preferably 6 to 12 carbon atomsand m is the valence of M, in an amount sufficient when activated tocure said epoxy resin admixed uniformly within said curable epoxy resin,wherein the microcapsule walls isolate the first curative from thesecond curative.

The exemplary activatable adhesive compositions referred to above may beused singly or in combinations of two or more compatible representativesthereof in the sheath of the adhesive article of the present invention.

The pressure sensitive adhesive which may be present in the sheath ofthe presently claimed adhesive article is preferably an acrylic basedpressure sensitive adhesive, but other pressure sensitive adhesives arecontemplated as well and may be used. Such other pressure sensitiveadhesives include for example those based on silicones or based onpolyolefins as disclosed in Handbook of Pressure Sensitive AdhesiveTechnology (third edition) D. Satas, Ed. Satas and Associates, WarwickRI/USA, 1989 on pages 550-556 and 423-442 respectively.

Particular examples of suitable pressure sensitive adhesives include,but are not limited to, adhesives based on general compositions ofpoly(meth)acrylate; polyvinyl ether; diene rubber such as naturalrubber, polyisoprene, and polybutadiene; polyisobutylene;polychloroprene; butyl rubber; butadiene-acrylonitrile polymer;thermoplastic elastomer; block copolymers such as styrene-isoprene andstyrene-isoprene-styrene (SIS) block copolymers,ethylene-propylene-diene polymers, and styrene-butadiene polymers;poly-alpha-olefin; amorphous polyolefin; silicone; ethylene-containingcopolymer such as ethylene vinyl acetate, ethylacrylate, and ethylmethacrylate; polyurethane; polyamide; epoxy; polyvinylpyrrolidone andvinylpyrrolidone copolymers; polyesters; and mixtures or blends of theabove. The pressure sensitive adhesive composition may contain additivesincluding, but not limited to, tackifiers, plasticizers, fillers,antioxidants, stabilizers, pigments, diffusing materials, curatives,fibers, filaments, and solvents.

Pressure sensitive adhesives that may be used in combination withactivatable adhesives to preliminarily bond to substrates having a lowsurface energy or oily surfaces, include for example pressure sensitiveadhesives based on an acrylic copolymer of one or more alkyl esters ofacrylic or mefhacrylic acid and a vinyl ester as disclosed in forexample EP 1318181 or a pressure sensitive adhesive as disclosed in EP 1245 656 which discloses a pressure sensitive adhesive composition thatcontains (i) the reaction product obtainable from a precusor compositioncomprising one or more alkyl esters of acrylic or methacrylic acid, oneor more copolymerizable monomers that have a Lewis base functionalityand optionally one or more cross-linkers and (ii) one or more tackifyingresins. Still further pressure sensitive adhesive that may beparticularly useful for adhesion to an oily surface are disclosed in WO95/13331.

Suitable commercially available pressure sensitive adhesives includethose commercially available from 3M Company under the designation VHB.

Generally, the manufacture of the adhesive article according to theinvention comprises the steps of: (i) providing a core comprising afibrous web and (ii) applying to the core a sheath, comprising anadhesive composition.

The adhesive compositions included in the sheath may be formulated as atape or film comprising at least one major adhesive surface. Theadhesive composition may also be present on both major surfaces of thetape or film. In such a form, the adhesive can be wound around the coreto provide the sheath of the adhesive article according to theinvention. Preferably, the adhesive composition(s) contained in the tapeor film has a sufficient initial level of tack at least on one side toadhere to the core and to provide a sheath/core structure which issufficiently stable for handling and application.

Alternatively, the adhesive compositions may be formulated as a fluidand may be coated as such on the surface of the core to provide a sheathwhich fully or partly covers the core. Suitable methods for coating arenot limited and include a method of applying a solution or suspension ofan adhesive composition, e.g. by spraying, roll coating, extrusion,co-extrusion, flow coating, microfiberisation, or by application ofbeads of a liquid or paste adhesive, etc. optionally accompanied orfollowed by a step of drying or further treatment such as heatprocessing or exposure to radiation to effect further chemical orphysical change in the applied composition.

As set out above, compressibility and/or flexibility of the adhesivearticle may be improved by structuring the sheath. For example, thesheath may be formed so as to extend along the surface of the core in adiscontinuous manner. Such a sheath may provide holes, cuts or otherapertures which may or may not extend fully around the perimeter of theadhesive article.

One embodiment of such a structured sheath would be a sheath comprisingseparate loops of the sheath material which extend fully around theperimeter of the adhesive article or at least around the major part ofthe perimeter. For example, such loops may conveniently be obtained bywinding strips of an adhesive composition formulated as a tape as setout above around the core in predetermined distances. The width of theloops and the width of the distances between them may be independentlychosen according to necessity. For example, the width of the tape mayrange from 5 to 30 mm and the distance between the loops may range from2 to 20 mm. The loops may be applied around the perimeter in a directionperpendicular to the longitudinal axis of the adhesive article, or theymay be inclined towards the longitudinal axis, resulting in continuousor discontinuous helical structures.

Thus, a particular embodiment of such a structured sheath would be asheath comprising at least one helix comprising an adhesive composition,the at least one helix extending in longitudinal direction along atleast a portion of the core. Also, the sheath may comprise at least onesecond helix comprising an adhesive composition which is counter woundto the first helix and which extends in longitudinal direction along atleast a portion of the core. Preferably, the at least one first helixand the at least one second helix extend in longitudinal direction atleast partially along the same portion of the core so as to form acounter wound double helix. Preferably, the helix/helices should leave acertain amount of space between the separate windings so that a part ofthe surface of the core remains uncovered. For example, such helices mayconveniently be obtained by winding strips formulated as a tape orapplication of beads of a liquid or paste adhesive as set out abovearound the core with a predetermined distance between the separatewindings. The width of the windings and the width of the distancesbetween them may be independently chosen according to necessity. Forexample, the width of a winding may range from 5 to 30 mm and thedistance between the windings may range from 2 to 20 mm.

Further methods of structuring the sheath to increase its flexibilityinclude the application of the sheath in varying thicknesses. Forexample, sections of the adhesive article having a thicker, lessflexible sheath may alternate with sections which carry a reduced amountof the sheath material. In a further embodiment, the sheath may comprisegrooves which are applied so as to facilitate bending of the adhesivearticle along its longitudinal axis.

It should be understood that the different methods of structuring thesheath referred to above are not mutually exclusive and can be used incombination in one adhesive article.

In addition to the core and the sheath, the adhesive article of thepresent invention may be provided with one or more release liners toprotect an outer surface of an adhesive layer. Typically, the releaseliner comprises a film or paper substrate coated with a releasematerial.

The method of activation to form a bond between the adhesive article anda substrate to which it is applied, if required, depends on the adhesivecomposition present in the adhesive layer and can be suitably selectedfor a variety of applications. Generally, the adhesive composition isactivated and bonds much more quickly and more uniformly than bulkadhesive systems, because there js very little mass of the adhesive.Only the very thin outer layer of adhesive cures/polymerizes and thisdoes not result in a reduced normal contraction force, which coulddamage the surface of the substrate. Thus, there is reduced read-throughdue to the fact that there is no net volume reduction because the bulkof the gap filling system does not cure/polymerise. Cure is alsocompletely uniform due to low mass.

For example in heat activatable adhesives, cure temperatures are reachedrapidly. In addition, there is typically no exofherm, so the probabilityof adhesive overheating or charring is negligible. In addition, if thecore materiel is porous, any residual liquids, such as pre-electrocoatwashing solutions present on parts to be bonded in the automotiveindustry, will drain rapidly away through the adhesive and any remainingmoisture will evaporate rapidly during heat activation.

The adhesive articles of the present invention are suitable for avariety of applications which require bonding, filling, dampening,sealing or stiffening properties.

Thus, the invention further provides a method of bonding comprisingbonding the adhesive article according to the invention to a substrate.Generally, in bonding applications, the adhesive article is bonded to afirst and second substrate so as to bond said first and second substrateto each other. Often, joining processes using the adhesive articlesaccording to the invention comprise the application of the article to asurface and then bringing a second surface in contact with it whilecompressing the joint to the correct height.

The adhesive articles according to the invention may be flexibly appliedand adapted, e.g., to corners. They have a beneficial gap fillingcapability, and may easily be adapted to gaps ranging between lowerlimits of 0.2 or even 0.1 mm and upper limits of 20 mm, 30 mm or even 40or 50 mm. The adhesive articles of the present invention have particularadvantages in joints with a non-uniform distance between the substratesto be joined. In such a case, the adhesive article may bridge andsupport an important gap at areas of higher distance. In areas ofreduced distance, the core is more compressed so that areas of higherbond strength are achieved.

For specific applications, it is possible to provide adhesive articlesof which the core can be removed, e.g. by the action of heat, after abond has been established to the first and the second substrate. Such aproperty could be used for example to provide hollow, lightweightadhesive structures or for recycling purposes.

It is also possible to apply the adhesive article according to theinvention for stiffening, dampening or insulation purposes. Thus, theinvention further provides a method of enhancing the stiffness of asubstrate, for dampening a substrate or to improve the insulativeproperties of a substrate comprising bonding the adhesive articleaccording to the invention to a substrate. It is possible to apply theadhesive article according to the invention to a single substrate or totwo or more substrates for these purposes.

On or between substrates, the adhesive articles according to theinvention may form a pattern which provides increased stiffness,dampening and/or insulative properties at a low weight, similar tohoneycomb cores. For example, a plurality of the adhesive articles,positioned parallel to one another or in any other desiredconfiguration, may form an array. Alternatively, the adhesive articlemay be positioned on or between substrates in more or less denseS-curves for the same purpose. Thus, for example a high stiffness may beobtained with minimal weight increase by using the adhesive articleaccording to the invention. The benefit of this is to enhanceperformance, reduce denting and dampen vibration e.g. on hoods, doors orroofs of transportation vehicles such as car and vans.

For example, the adhesive article according to the invention can be usedto bond a variety of substrates, such as metal sheets or panels, plasticor composite materials. In a particular embodiment, the adhesive articleis used to bond to a component of a transportation vehicle, such as anaircraft, boat, bus or train, in particular to a component of a motorvehicle such as a car, a bus, a truck or other motor vehicle for use ona road. In a further embodiment, the adhesive article is used to bondcomponents of such a transportation vehicle, in particular motor vehiclefor use on the road, together. Still further, the adhesive article maybe suitably used for gap filling purpose in the aforementioned vehicles.

Application in the form of an adhesive article ensures easy and safehandling compared, e.g. to the application of a liquid or paste adhesiveformulation.

The adhesive article according to the invention can be applied to asubstrate, such as a sheet or a panel of metal, plastic or of acomposite material, and the fibrous web can be rendered stiff by any ofthe methods set out above, such as fiber linkage, melting or any otherform of chemical cure. The effect of such an adhesive article would beto further stiffen the substrate to which it is applied.

Finally, the property of high stiffness but easy deformation underexcessive loads which may be achieved with the adhesive article of thepresent invention can be useful for example in situations where avehicle impacts a pedestrian. Using a properly selected adhesive articleaccording to the invention as a stiffener, day-to-day normal panelstiffness is retained, but in case of impact higher levels ofdeformation may be achieved.

EXAMPLES Materials Used for the Example Preparation a) Structural EpoxyFilm Adhesive

As the heat activatable adhesive, use was made of a cross-linkableadhesive film SAF 6045 commercially available from the 3M Company (St.Paul, Minn./USA). SAF 6045 is a modified epoxy film and has a nominalthickness of 0.4 mm. Typical overlap shear values according to ASTM1002-94 measured initial at 23° C., after having the sample cured at165° C. for 15 minutes plus 10 minutes oven ramp, are, depending on thesubstrate, between 15.3 MPa and 19.0 MPa.

b) Fibrous Web Made by Vertical Cross-Lapping Technology (300 g/m²Weight)

The fibrous web used was made up of 15 den polyethyleneterephtalate(PET) staple fibres, available from Inquitex SA/Spain under thecommercial name PET fibre type 101. The typical length of the PET staplefibres was between 38 to 42 mm, and the fibres had an average elongationof 80%. The web further comprised thermal bond fibres, also known asbicomponent (BICO) fibres. The bicomponent fibres were core-sheathfibres, composed of a copolymer polyethylene terephtalate/polyethyleneisophtalate (sheath) and a homopolymer polyethylene terephtalate (core)having an average staple length of 50 mm, commercially available byTangerding, Germany.

For manufacturing the non-woven web, small batches (100 g) of the fibreblend—comprising of 80% 15 den PET fibres and 20% thermal bond BICOfibres—were mixed by hand. These batches were then hand fed into asingle cylinder card at a constant rate. The carded web was thencross-lapped and lightly needle punched and then folded in half alongits length. This pre-web was then used to feed a second single cylindercard, which provided the web used that is fed into the vertical crosstapper (VCL).

The vertical cross-lapping process was run on a Struto machine(developed in the Czech Republic at the University of Liberec) set up togive the desired thickness and the thermal bond oven set at 140° C. witha double belt collector. The Struto process uses a bar to force thefibres into a vertical orientation. Such a process pushes the ends ofthe fibres at the point when the direction is changed out of the loopgiving a highly oriented product in the z-direction.

For test assembly preparation, one-side ground steel panels (availableas RS/14 from Q-Panel Company Express Trading Estate, Farnworth, Bolton,BL49TP, U.K.) were selected. The panels were 1.6 mm thick and haddimensions of 10.2 mm by 15.2 mm. No cleaning was conducted prior toassembling the panels with the tubular adhesives. Accordingly, thepanels were used as supplied, having oil on them which is applied inorder to avoid corrosion occurring. The oil tends to evaporate in theoven bake cycle at 180° C.

Example Preparation Example 1

The previous described vertical cross-lapped fibrous web strips (2 cmthick, 2 cm width) were selected as the core material, being composed of80% 15 den PET fibres and 20% 4.4 den Bico thermal bond fibres.

These fibrous web strips were wrapped on their outsides with the heatactivatable adhesive film, using a non-overlapping technique to yield ahelix of the adhesive tape.

The material was then placed between two 1.6 mm thick one-side groundsteel panels. The so acquired test assembly was then pushed together byhand and conditioned at 23° C. and 50% relative humidity for 24 hoursbefore testing.

After conditioning the test assembly was placed in a forced air oven andthe temperature was raised to 180° C. according to the followingprogrammed profile:

-   -   The oven temperature was ramped up from 23° C. in 10 minutes to        180° C.    -   The oven temperature was then held at 180° C. for 30 minutes    -   After 30 minutes the oven was turned off, the oven door opened        and the sample allowed to remain in the oven until cooled out.        After trie test assembly removal from the oven the curing cycle        of the adhesive was seen as completed

The cured test assembly was conditioned at 23° C. and 50% relativehumidity for 24 hours prior to testing.

Example 2

A second sample material was prepared using the same VCL fibrous web asdescribed in example 1, but by wrapping the cross-linkable film adhesivein a counter-wound double helix structure around the core material.Hereby the strip of the non-woven fabric was cut from the VCL sheet bymaking a cross web cut, axial with corrugation. The cut section was 20mm wide and 200 mm long. The 3M SAF 6045 epoxy film adhesive(commercially available from the 3M Company, St. Paul, Minn./USA) wasthen applied by winding the tape in a helical pattern using a pitch of24 mm leading edge to leading edge. The winding procedure was repeatedby using the same tape but applied in the opposite direction to thefirst tape. This created a cylinder epoxy adhesive wound in a crossoverpattern around the flexible non-woven core. The resultant diameter ofthe non-woven tube was approximately 16 mm.

The counter-wrapped double helix structure was highly flexible andcapable of high conformance.

The so-formed tubular stiffener was applied length-wise to a thin,one-side brushed steel panel of the following dimensions; 102 mm×152mm×0.254 mm, of type brushed one side supplied by the Q-Panel Company.This panel was used as received without any additional surfacepreparation and the adhesive was applied to the brushed surface. Afterconditioning the test assembly at 23° C. and 50% relative humidity for24 hours it was placed in a forced air oven and cured 20 minutes at 176°C. without any specific ramp up time. The oven was at temperature whenthe sample was placed in it and there was no programmed ramp up totemperature.

In this cured state example 2 material turns very strong and stiff,providing a tubular stiffening system suitable for single-sidedapplications such as replacement of top hat stiffeners or more expensivedoublers against stressed skin.

3-Point Bend Test

A 3-point bend test was performed with the adhesive article of example2.

For performing this test a counter wound adhesive article according toexample 2 was length-wise bonded centrally along the long axis of thesteel panel. The panel was supported on aluminium trestles on both sidesof the adhesive and the trestles had a height of 30 mm. The contact withthe panel was 30 mm×3 mm on each side of the adhesive article. The samewas done on the other side of the panel so that it was like a tablesupported on four 30 mm edges at right angles to the long axis of thepanel. These supports were then placed 140 mm apart thus leaving a 140mm span between trestles. Now the central portion of the panel waspushed down to measure the force to deflect. This was done with a flatMDF probe of end dimensions 37 mm×8 mm placed in the centre of the panelwith the 37 mm length at right angles to the long axis of the panel. Theprobe was now moved down at constant speed (1 mm/minute) and the forcerequired measured in compression using normal tensometry. Test resultsare given in Table 1 below.

TABLE 1 With adhesive Control Panel deflection article as stiffener (nostiffener) (mm) (force (N) (force (N) 0 0 0 1 13 0.6 2 22 1.3 3 37 2.0 450 2.7 5 61 3.3

A strong structural bond was created in examples 1 and 2. The results ofthe 3-point bend test performed with example 2 additionally showed aclear improvement in panel stiffness up to a 6 mm deflection. It isinteresting to note that the adhesive article used to achieve thisincrease in stiffness weighed only 6.58 g. This enabled the plate tocarry a load of nearly 70N (7000 g) before failure. Up to the breakpoint, the panel stiffness was increased by a factor of 17 for theselected panel thickness.

1.-27. (canceled)
 28. An adhesive article comprising an elongated corecomprising a fibrous web, said core being provided with a sheathextending at least around a major part of a core perimeter, said sheathcomprising an adhesive composition having as outer adhesive surface,wherein the smallest diameter of the core is 1 mm or above.
 29. Adhesivearticle according to claim 28, wherein the adhesive article has across-sectional shape perpendicular to its longitudinal axis, saidcrass-sectional shape comprising a circular shape, a polygonal shape, oran elliptical shape.
 30. Adhesive article according, to claim 28,wherein the adhesive article is flexible along its longitudinal axis.31. Adhesive article according; to claim 28, wherein the sheath extendsdiscontinuously along the surface of the core.
 32. Adhesive articleaccording to claim 31, wherein the sheath comprises holes, cuts or otherapertures.
 33. Adhesive article according to claim 28, wherein thesheath extends completely around the core perimeter.
 34. Adhesivearticle according to claim 28, wherein the sheath comprises at least onehelix extending in a longitudinal direction along at least a portion ofthe core longitudinal axis.
 35. Adhesive article according to claim 28,wherein the sheath comprises a first helix and a second helix extendingin a longitudinal direction along at least a portion of the core, saidfirst and second helix forming a counter wound double helix along atleast a portion of the core.
 36. Adhesive article according to claim 28,wherein the sheath fully covers the core.
 37. Adhesive article accordingto claim 21, wherein the core comprises a non-woven fibrous web. 38.Adhesive-article according to claim 28, wherein the sheath comprises anactivatable adhesive, a pressure sensitive adhesive, or both. 39.Adhesive article according to claim 28, wherein the smallest diameter ofthe core is 6 mm or above.
 40. Adhesive article according to claim 28,wherein, said adhesive, article can be reversibly compressed by at least10% along the thickness direction.
 41. Adhesive article according toclaim 28, wherein said adhesive article can be reversibly compressed byat least 30% along the thickness direction.
 42. Adhesive articleaccording to claim 28, wherein said adhesive article can be reversiblycompressed by at least 70% along the thickness direction.
 43. Method ofmanufacturing an adhesive article as defined in claim 28 comprising (i)providing a core comprising a fibrous web and (ii) applying to the corea sheath comprising an adhesive composition.
 44. Method of bondingcomprising bonding the adhesive article as defined in claim 28 to atleast one first and at least one second substrate so as to bond saidfirst and second substrate to each other.
 45. Method according to claim44 wherein said first and second substrate define between them a gap ofbetween 0.2 mm and 20 mm and wherein said adhesive article fills saidgap.
 46. Method of enhancing the stiffness of a substrate comprisingbonding the adhesive article as defined in claim 28 to the substrate.47. A first substrate bonded to the adhesive article of claim 28.